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Then he had a stroke of luck when a caretaker brought in a metal tin with the words 'zoology practical' unwisely written on the side.

Sir David told the Leicester Mercury: 'When I got the chance I picked it up and shook it. It
rattled so I knew it was crayfish.

'I knew where I could get crayfish -
the stream at [nearby] Bradgate Park, so I headed there. I told all the other
boys.'

They then fished crayfish out of the stream and practiced dissection on them to finesse their skills.

The move clearly paid off. Not only did Sir David and his friends win distinctions in the exam - which was the equivalent of a modern A-level exam - but their teacher also said they were the most brilliant class he had ever had.

Sir David made the confession as he revisited the park, a former Medieval deer hunting ground which is now run by a charitable trust.

He was there to help the local Rotary Club launch an appeal to raise £150,000 to turn an old barn into a visitor centre.

There was good reason for choosing the TV naturalist for the event - as Sir David's father was once the president of the Rotary Club of Leicester.

He told the BBC: 'Bradgate is a very important part of my childhood... We came here as a family up onto those rocks, onto the tower, paddling in the stream.

'It was a vision of what the English countryside could be like and it's stayed with me forever.'